Thermoelectric generator



Jan. 22, 1963 R. A. ELM ETAL 3,075,030

THERMOELECTRIC GENERATOR Filed Deo. 22, 1959 3 Sheets-Sheet 1 l 42 f f ROBERT A. ELM WILLIAM G. KRAWCZAK DELMAR D. SCHLEY EVERT J. LEVIN Jan. 22, 1963 Filed Dec. 22, 1959 R. A. ELM ETAL.

THERMOELECTRIC GENERATOR 3 Sheets-Sheet 2 FIGZ INVENTORS ROBERT A. ELM WILLIAM G. KRAWCZAK DELMAR D. SCHLEY EVERT J. LEVIN Jan. 22, 1963 R. A. ELM ETAL 3,075,030

THERMOELECTRIC GENERATOR Filed Dec. 22, 1959 3 Sheets-Sheet 3 INVENTORS ROBERT A.ELM WILLIAM G. KRAWCZAK DELMAR D. SCHLEY EVERT J. LEVIN ATTORN YS 35375936 THERMGELEC'ERQ GENERATR Robert Ac Elm, St. Paul, and William G. lrawczak, Delmar D. Schley, and Evert `l. Levin, White Bear Lake, Minn., assignors to Minnesota Mining and Manufacturing Company, St. Paul, Minn., a corporation of Deia- Ware Filed Dee. 22, 1959, Ser. No. 861,323 13 Claims. (Cl. 13o-nd) This invention relates to improvements in thermoelectric generators and relates more particularly to thermopiles of improved eiciency.

rl`he development in recent years of improved thermoelectric materials in the form of semiconductors having high thermoelectric power, low electrical resistivity, and low thermal conductivity has created renewed interest in the development of thermoelectric generators producing usuable amounts of power and having efficiency characteristics heretofore unobtainable. These new materials pose certain problems, however, since they are extremely fragile and tend to oxidize as well as to sublimate at higher temperatures.

it is the general object of the present invention to provide an improved thermoelectric generator in which the use of improved thermoelectric materials is complemented by the embodiment or" said materials into a generator structure characterized by high thermal eiiciency and compactness.

Another object of the invention is to provide an improved thermoelectric generator having embodied therein adjustable connections aifording a thermoelectric circuit of consistently low electrical resistance as well as affording efficient heat transfer from the heat source to the hot thermojuuctions and from the cold thermoiunctious to the heat dissipating means, the adjustable character of said connections making it unnecesary to maintain extremely fine tolerances such as would otherwise be required or optimum thermal transfer.

Another object of the invention is to provide an improved thermoelectric generator of the aforementioned character which is well adapted for use with a variety of heat sources.

Still another object of the invention is to provide an improved generator of the class described affording stable and consistent operation by virtue of incorporation therein of means for eliminating or reducing to tolerable limits both sublimation and oxidation of the thermoelements.

Another object of the invention is to provide an irnproved thermoelectric generator affording maximum utiization of the heat input by virtue of the incorporation therein of novel means tending to prevent escape of heat from the heat source except through the thermoelernents.

Another obiect of the invention is to provide an improved thermoelectric generator in which the heat source is capable of being supported substantially and entirely by the thermoelements which are placed under longitudinal compression to afford a low resistance pressure contact at the hot thermojunctions and to afford the thermoelements substantial shock resistance, said longitudinal compression being applied to the thermoelernents by novel heat transfer and force applying means of a character to minimize any bending moments in said thermoelements.

Another object of the invention is to provide an improved thermoelectric generator which is disposed within an hermetcally sealed heat conductive enclosure, said enclosure being operative to function as heat dissipating means and being isolated from any mechanical stresses incident to placement of the thermoelements under longitudinal compression.

Other and further objects and advantages of the invention will become apparent as the description proceeds,

hired tats atet E reference being had to the drawings accompanying and forming a part of this specification in which:

FIGURE 1 is a view partly in elevation and partly in vertical section illustrating one form of thermoelectric generator constructed in accordance with the inventive concept, the vertical section being taken approximately along the line l-i of FlGURE 2;

FIGURE 2 is a plan View of the improved generator of FIGURE l with the cover and a portion of the thermal insulation removed, parts being broken away and shown in section; and

FIGURE 3 is a view similar to FlGURE 1 illustrating another form of thermoelectric generator constructed in accordance with the inventive concept,

Referring more particularly to FiG'URE l of the drawings, the form of the invention illustrated therein comprises an hermetically sealed metallic enclosure 5 of good thermal conductivity including a cylindrical intermediate section 6 having thickened end portions 7 and 8 grooved to sealingly receive generally hemispherical end caps 9 and lll which are sealingly fixed within the accommodating grooves by soldering. A cylindrical heat dissipating and supporting member il of metal of good thermal conductivity has a snug coaxial heat conductive fit within the enclosure section 6 as shown. Bisposed coaxially of and in spaced relation with the annular member 1l is hot thermojunction means comprising a metallic sleeve i2 of good thermal conductivity having a plurality of axially extending rectangular peripheral grooves 13. Nine such grooves i3 are shown in PiQURE 2. The hot thermojunction means also includes a plurality of rectangular hot junction electrodes or shoes lli each having a snug heat conductive lit within the accommodating groove i3.

It will be observed that in the form of the invention illustrated in FGURE l there are three hot junction electrodes 3M in each groove i3. In order to electrically insulate the electrodes ld from the sleeve l2 while providing a connection of good thermal conductivity therebetween, the surfaces of the electrodes ld engaging the surfaces of the grooves i3 are provided with a thin layer of insulating material which may take the form of an aluminum oxide coating applied thereto, for example, by ame spraying. Alternatively, a thin layer or" mica may be provided as an interface between the mating surfaces of the electrodes ld and grooves lli. The hot junction electrodes i4 are preferably made of iron, and the outer surface of each of said electrodes is formed with a pair of spaced axially aligned flat-bottomed cylindrical recesses l5 and lo. rlhe axes of the recesses l and lo are in parallel planes and for a given electrode i4 are preferably disposed at an angle to one another symmetrically about a radial plane extending along the axial center line of the electrode.

A plurality Lof generally radially extending cylindrical thermoelements ll of one conductivity type. for example N-type, have their inner ends disposed coaxially within the respective recesses i5 of the electrodes le, said thermoelements being preferably surrounded by a snug fitting sleeve 13 of mica or other suitable material, and said tube having a snug fit within the respective recess l5. A plurality of generally radially extending cylindrical thermoelements i9 of opposite conductivity type, for example P-type, have their inner ends coaxially disposed within the respective recesses lid and are preferably snugly surlrounded by coaxial sleeves 2li of mica or other sui-table material which, in turn, has a snug lit within the respective recess lo.

The thermoelements 17 and i9 may be of any suitable thermoelectric material having a high thermoelectric power, low electrical resistivity and low thermal conducenv-5,030

tivity. Preferred materials include those disclosed in Robert W. Fritts and Sebastian Karrer United States Letters Patent No. 2,811,571. For maximum thermal conversion efficiency. it is preferred to have the thermoelements l? Aand Z919 formed of segments o-r portions of thermoelectric material affording an optimumV ligure of merit for the particular portion of the temperature gradient in which the segment or portion operates. Thermoelements of this type are disclosed in the copending application for United States Letters Patent of Russell E. Fredrick and Robert W. Fritts, Serial No. 775,529, led November 2l, 1958, now US. Patent No. 3,051,767, assigned to the assignee `of this application.

As shown in FEGURES l and 2, the annular heat dissipating and supporting member ll istforrned with a separate radial bore 2l coaxially aligned with each of the therrnoelements 1'?, and said member is further formed with a separate radial bore 21?; coaxially aligned with each of the thermoelements 19. Electrical contact with the thermoelements '117 is made by cold junction electrodes 2,5, formed, for example, of copper. Each-of the electrodes 23 is formed with a cylindrical recess 24 into which the outer end of the associated thermoelernent 17 is fixed, as by soldering. Electrical contact with the thermoelements 19 is made by cold junction electrodes 25 which are similar to the electrodes Z3 and are similarly fixed to the outer ends of the thermoelements .19. The cold junction electrodes 23 and 25 are disposed within the inner ends of the bores 2l and 22 respectively as shown.

Means is provided for placement of the thermoelements f7 and i9 under longitudinal compression, said means being effective to bias said therrnoelements into low electrical resistance pressure contact with the associated hot junction electrodes or shoes 114 as Well as to afford said thermoelements substantial shock resistance. To this end, the cold junction electrodes 23 and 25 are each provided with an outer surface of generally spherical configuration. A cylindrical force applying member 26 is disposed within each of the radial bores 2l and 22 and has a generally spherical inner end surface complemental to the outer surface `of the associated cold junction electrode 23 or 2S as shown. The outer end surface of each of the force applying members 2o is recessed as at 27 to receive a helical compression spring 23, the outer end of which bears against an adjustment screw 29 threaded into the outer end of the accommodating bore 21 or 22. The compression springs 2, acting through the force applying members 2o exert a radially inwardly directed force on,

the cold junction electrodes 23 and 25 to thereby exert a similar force on the thermoelernents 17 and i9, biasing the inner ends of said thermoelemen-ts into low resistance electrical contact with the hot junction electrodes 14 within the recesses l and 16. This force also biases the hot junction electrodes ld into engagement with the base of the accommodating grooves i3 of the sleeve l2 to afford good thermal transfer from said sleeve to said thermoelements.

The connection between the cold junction electrodes 23 and 25 and the associated force applying members 26 is a generally ball and socket type of connection which is particularly advantageous. Mechanically, it insures that the longitudinal compressive stress is applied to the thermoelements in a manner to minimize any bending moments in said ithermoelements. Also, said connectionV permits relative movement `of the parts during heating up and cooling down, thereby protecting the therinoelements from bending stresses which might otherwise be impressed thereon `during such times. Further, the relatively large interface between the cold junction electrodes 23 and 25 andthe associated force applying members 26 insures good heat transfer from the thermoelernents to said force applying member, whereas the VYrelatively large interface between said force applying members and the heat dissipating member il insures goed heat transfer to the member ill from which the heat hows to the enclosure 5 for transfer to the environment. The adjustment screws'I 29 permit the force exerted on the therrnoelements by thel springs 2? to be individually adjusted, and such adjust- `ability permits the use of less precise tolerances than would otherwise be required if equivalent thermal transfer were to be attained in the absence of such adjustability.

Each hot junction electrode or shoe i4 and the particular thermoelements 17 and 19 associated therewith forms a thermocouple, there being in the illustrated embodiment three superimposed radial arrays of nine thermocouples each. As shown in FGURE 2, the thermocouples of each radial array are connected in series circuit by jumpers or conductors Si@ which connect the cold junction electrode 23 of one the-rmocouple with the cold junction electrode 2' of an adjacent thermocouple in the same array. Jumpers or conductors 31 connect the superimposed radial arrays in series circuit by connection ofAA a contact electrode 25 of one array with a contact electrede 23 of the subjacent radial array. Power output con-- nections for the thermoelectric generator including all ofI the thermocouples (FIGURE 1) connected in series cir-- cuit are provided by a conductor 32 connected to a coldjunction electrode 25 of the lowermost array constitutingi one end of the series circuit, the conductor 32 being` brought out through a suitable hermetic seal 33 in the' enclosure d. Another conductor 34 (FEGURE 2) is connected to a cold junction electrode Z3 of the uppermost array constituting the other end of the series circuit, the conductor 34 being similarly brought out through a suitable seal (not shown) in the enclosure 5. The conductors 32 and 34 are adapted for connection to any suitable load, preferably one having an electrical resistance matched to that of the illustrated generator.

in order to prevent electrical short circuiting of the therrnocouples to the supporting and heat dissipating member 11 all of the surfaces of the force applying cylinders 26 except the concave surface engaged by the associated coldjunction electrodes 23 or 25 are provided with a thin coating of aluminum oxide. Such an aluminum oxide coating affords electrical insulation while at the same time having good thermal conductivity permitting eiicient heat transfer between the cylindrical surfaces of the members 25 and the surrounding bore surfaces of the member 1l. Heat transfer from the cold junction electrodes 23 and 25 to the member il is enhanced by the use of a thin film of high vacuum grease of good thermal conductivity, for example silicone grease, at the interfaces between the cold junction electrodes and the force applying members 26, as well as at the cylindrical interfaces of the members 26 and the surrounding surfaces of the accommodating bores 2l and 22.

The sleeve 112 is formed with a tapered bore, for example a bore 35 having a 4 degree locking taper, and disposed within said bore is a capsule-or cup-shaped member 3o having a frusto-conical outer surface complemental to the bore 35 and having a snug it therein. The:

capsule 35 is preferably made of metal having a higher coeiiicient of thermal expansion than that of the material from which the sleeve l2 is made. For example the. capsuley 36 may be made of austenitic stainless steel and the sleeve lo of ferritic stainless steel. This insures that; as the temperature of the capsule 36 and sleeve 12 increases, the frictional engagement therebetween becomes more intimate and the resistance of thermal transfer at the interface decreases. Preferably the capsule 36 is cooled prior toinsertion into the sleeve 12. The capsule 36 accommodates a heat source which may be of any suitable cr desired type. ln the illustrated embodiment the heat source takes the form of a quantity 37 of radioactive isotope sealed within concentric cylinders 38 and 39, the latter being disposed within a bore 40 in capsule 3o as shown.

In order to minimize the escape of heat from the heat source-except through the thermoelements 17j and 1,9, all

of the unused space within the enclosure 5 is lled with thermal insulation. In the form of the invention shown in FIGURES 1 and 2 the efficiency of the generator is enhanced by using concentric segments 41 and 42 of block type insulation each of which affords optimum thermal insulating characteristics within a different temperature range. For example, the inner segment or shell 41 may be formed of colloidal silica, and the outer segment or shell 42 may be made of fibrous silica. The colloidal silica aords optimum thermal insulation characteristics at the higher hot thermojunction temperature range to which it is exposed, and the iibrous silica alfords optimum thermal insulation characteristics at the cold thermojunction temperature range to which it is exposed. It will be observed that the lower portion of the inner segment 41 abuts the lower end of the sleeve 12 and that the upper portion thereof abuts the upper end of the capsule 36 so that the capsule 36 is maintained firmly seated within the bore 35 of the sleeve i2 at all times. The effectiveness of the thermal insulation 41 and 42 is enhanced by removal of any air from the enclosure 5, for example through the tubes d3 and 4d, and the substitution therein of a high molecular weight gas, for example argon, containing a trace of a reducing gas, for example methane, after which substitution the tubes 43 and 44 are sealed.

The use of the reducing gas within the enclosure 5 tends to prevent oxidation of the thermoelements 17 and I9, particularly at the pressure contact thereof with the hot junction electrodes i4 where the formation of oxide lilms would tend to increase the electrical resistance of the circuit. The snug fit of the mica tubes 18 and 20 around the thermoelements 17 and 19 respectively and within the recesses 15 and 16 respectively tends to prevent or restrict sublimation of the thermoelements at the hot iunction end. The tubes IS and Ztl further function to support the thermoelements against deformation due to transverse acceleration or shock.

lt will be observed that the enclosure 5, in addition to functioning as an hermetically sealed container for the thermoelectric generator, functions as an etlicient heat radiating and convecting means. If desired, the enclosure 5 may be provided with cooling tins or other suitable additional heat dissipating means in any well known manner. The container 5 need not be constructed to withstand substantial pressures, since it is not subjected t any mechanical stresses incident to the placement of the thermoelements under longitudinal compression, any such mechanical stresses being isolated from the enclosure by the supporting and heat dissipating member 11 which is sufficiently strong to withstand such forces without deformation.

FIGURE 3 illustrates another form of thermoelectric generator constructed in accordance with the invention and is identical in most respects to the generator shown in FIGURES l and 2. In FIGURE 3 the elements indicated by primed reference characters to correspond to similar elements shown in FIGURES l and 2 and indicated by the same reference characters unprimed. In FIGURE 3, the thermal insulation 41 and 42 of the generator shown in FIGURES l and 2 is replaced by rellector means. This reflector means takes the form of a pair of concentric reiiectors 45 and 46 having metallic highly polished inner and outer surfaces. The rellector 45 comprises a tubular cylindrical intermediate section 47 and end caps 48 and 49, the section 47 thereof being perforated to permit the passage therethrough of the thermoelements i7 and 1% and their surrounding mica sleeves. As shown the intermediate section 47 is spaced from the sleeve i2. The reflector 46 comprises a tubular cylindrical intermediate section 50 and opposite end caps 51 and 52., the intermediate section 5t) thereof also being perforated to permit the passage therethrough of the thermoelements i7 and i9 and their surrounding mica sleeves. As shown the reilector 46 is spaced outwardly d from the reflector 45 with the intermediate section Sil being spaced between the section 47 of reflector 45 and the cold junction electrodes of the thermoelements.

The highly polished inner surfaces of the reflectors 45 and 46 relect back toward the hot thermojunction means and the heat source, heat radiated outwardly therefrom and tending to bypass the thermoelements. The highly polished outer surfaces of the reliectors 45 and 46 afford said reflectors low emissivity tending to reduce radiation of heat therefrom.

It will be observed that in FIGURE 3 all of the support for the central structure comprising the heat source and the hot thermojunction means including sleeve l2 and hot junction electrodes 14', as well as all of the support for the reflectors 45 and 46, is provided by the thermoelements I7 and i9. The insulation aiorded by the relleetors d5 and 46 is enhanced by the fact that the enclosure 5 is evacuated prior to the sealing oit or" the tubes 43 and 44', such evacuation concomitantly reducing the tendency of the therrnoelements to oxidize by removal of substantially all available oxygen from the enclosure 5'.

Having thus described two specific embodiments of the invention, it is to be understood that the illustrated forms were selected to facilitate the disclosure rather than to limit the number of forms which the invention may assume. Various modiiications, adaptations and alterations may be applied to the speciiic form shown to meet the requirements of practice, without in any manne-r departing from the spirit or scope of the invention, and all of such modica-tions, adaptations and alterations are contemplated as may come within the scope of the appended claims.

What is claimed as the invention is:

1. A thermoelectric generator comprising a thermally conductive annular heat dissipating and supporting member formed with a plurality of cylindrical radial bores, thermojunction means spaced from and centrally disposed within said annular member, a plurality of radially extending thermoelements spanning the annular space between. said thermojunction means and said annular member and each in registration with one of said radial bores, a junction electrode at the outer end of each of said thermoelemen-ts, a separate cylindrical thermally conductive force applying member having a snug coaxial sliding fit within each radial bore with which a thermoelement is in registry, and biasing means cooperable with said annular member and with each of said force applying members for causing the latter to engage and exert radially inwardly directed forces on said junction electrodes to thereby place each of said thermoelements under longitudinal compression.

2. A therrnoelectric generator according to claim l in which heat transfer between said force applying mem ber and said heat dissipating member at said slidable connection is enhanced by a layer of heat conductive grease at the interface therebetween.

3. A thermoelectric generator according to claim 1 wherein heat transfer between said junction electrodes and said annular heat dissipating member is enhanced by a layer of heat conductive grease on the surfaces of said force applying members engaging said electrodes and said heat dissipating member.

4. A thermoelectric generator comprising a thermally conductive annular heat dissipating and supporting member formed with a plurality of cylindrical radial bores, thermojunction means spaced from and centrally disposed within said annular member, a plurality of radially extending thermoelements spanning the annular space between said thermojunction means and said annular member and each in registration with one of said radial bores, a junction electrode at the outer end of each of said thermoelements, a separate cylindrical thermally conductive force applying member having a snug coaxial sliding it within each radial bore with which a thermoelement is in 

1. A THERMOELECTRIC GENERATOR COMPRISING A THERMALLY CONDUCTIVE ANNULAR HEAT DISSIPATING AND SUPPORTING MEMBER FORMED WITH A PLURALITY OF CYLINDRICAL RADIAL BORES, THEREMOJUNCTION MEANS SPACED FROM AND CENTRALLY DISPOSED WITHIN SAID ANNULAR MEMBER, A PLURALITY OF RADIALLY EXTENDING THERMOELEMENTS SPANNING THE ANNULAR SPACE BETWEEN SAID THERMOJUNCTION MEANS AND SAID ANNULAR MEMBER AND EACH IN REGISTRATION WITH ONE OF SAID RADIAL BORES, A JUNCTION ELECTRODE AT THE OUTER END OF EACH OF SAID THERMOELEMENTS, A SEPARATE CYLINDRICAL THERMALLY CONDUCTIVE FORCE APPLYING MEMBER HAVING A SNUG COAXIAL SLIDING 